The use of optical indicia, or bar code symbols, for product and article identification is well known in the art. Presently, various types of bar code symbol scanners have been developed. One common type of bar code symbol reader is the laser-based scanner, which uses a focused laser beam to sequentially scan the bars and spaces of a bar code symbol to be read. The majority of laser scanners in use today, particular in retail environments, employ lenses and moving (e.g., rotating or oscillating) mirrors and/or other optical elements in order to focus and scan laser beams across bar code symbols during code symbol reading operations.
In demanding retail scanning environments, it is common for such systems to have both bottom and side-scanning windows to enable highly aggressive scanner performance, so the cashier need only drag a bar-coded product past these scanning windows for the bar code to be automatically read with minimal assistance of the cashier or checkout personal. Such dual scanning window systems are typically referred to as “bioptic” laser scanning systems as such systems employ two sets of optics—a first set disposed behind the bottom or horizontal scanning window, and a second set disposed behind the side-scanning or vertical window.
In general, prior art bioptic laser scanning systems are generally more aggressive that conventional single scanning window systems. For this reason, bioptic scanning systems are often deployed in demanding retail environments, such as supermarkets and high-volume department stores, where high check-out throughput is critical to achieving store profitability and customer satisfaction. While prior art bioptic scanning systems represent a technological advance over most single scanning window system, prior art bioptic scanning systems in general suffered from various shortcomings and drawbacks.
In particular, the laser scanning patterns of such prior art bioptic laser scanning systems are not optimized in terms of scanning coverage and performance, and the scanning systems are generally expensive to manufacture by virtue of the large number of optical components presently required to constructed such laser scanning systems.
Additionally, in scanning a bar code symbol and accurately produceing digital scan data signals representative of a scanned bar code symbol, the performance of such aggressive laser scanning systems is susceptible to noise, including ambient noise, thermal noise and paper noise. During operation of a laser scanning system, a focused light beam is produced from a light source such as a visible laser diode (VLD), and repeatedly scanned across the elements of the code symbol. In the case of bar code scanning applications, the elements of the code symbol consists of a series of bar and space elements of varying width. For discrimination purposes, the bars and spaces have different light reflectivity (e.g., the spaces are highly light-reflective while the bars are highly light-absorptive). As the laser beam is scanned across the bar code elements, the bar elements absorb a substantial portion of the laser beam power, whereas the space elements reflect a substantial portion of the laser beam power. As a result of this scanning process, the intensity of the laser beam is modulated in accordance with the information structure encoded within the scanned bar code symbol.
As the laser beam is scanned across the bar code symbol, a portion of the reflected light beam is collected by optics within the scanner. The collected light signal is subsequently focused upon a photodetector within the scanner which, in one example, generates an analog electrical output signal which can be decomposed into a number of signal components, namely: a digital scan data signal having first and second signal levels, corresponding to the bars and spaces within the scanned code symbol; ambient-light noise produced as a result of ambient light collected by the light collection optics of the system; thermal noise produced as a result of thermal activity within the signal detecting and processing circuitry; and “paper” or substrate noise, which may be produced as a result of the microstructure of the substrate in relation to the cross-sectional dimensions of the focused laser scanning beam, or noise related to the bar code printing quality (e.g., bar code edge roughness, unwanted spots, void defects, and/or printing contrast).
The analog scan data signal has positive-going transitions and negative-going transitions which signify transitions between bars and spaces in the scanned bar code symbol. However, a result of such noise components or operating the scanner near the operational limits of the focal zones, the transitions from the first signal level to the second signal level and vice versa are not perfectly sharp, or instantaneous. Consequently, it is sometimes difficult to determine the exact instant that each binary signal level transition occurs in the detected analog scan data signal.
The ability of a scanner to accurately scan an encoded symbol character and accurately produce digital scan data signals representative of a scanned bar code symbol in noisy environments depends on the depth of modulation of the laser scanning beam. The depth of modulation of the laser scanning beam, in turn, depends on several important factors. Among the factors are (i) the ratio of the laser beam cross-sectional dimensions at the scanning plane to the width of the minimal bar code element in the bar code symbol being scanned; (ii) the signal-to-noise ratio (SNR) in the scan data signal processor at the stage where binary level (1-bit) analog to digital (A/D) signal conversion occurs; (iii) the object distance; and (iv) the field of view (FOV) angle.
As a practical matter, it is not possible in most instances to produce analog scan data signals with precisely-defined signal level transitions. Therefore, the analog scan data signal must be further processed to precisely determine the point at which the signal level transitions occur. Various circuits have been developed for carrying out such scan data signal processing operations. Typically, signal processing circuits capable of performing such operations include filters for removing unwanted noise components, and signal thresholding devices for rejecting signal components which do not exceed a predetermined signal level. One drawback to these approaches is that thermal and “paper” (or substrate) noise imparted to the analog scan data input signal tends to generate “false” positive-going and negative-going transitions in the first derivative signal, and may also generate zero-crossings in the second-derivative signal. Consequently, the circuit logic allows “false” first derivative peak signals and second-derivative zero-crossing signals to be passed on, thereby producing erroneous binary signal levels at the output stage of the signal processor. In turn, error-ridden digital data scan data signals are transmitted to the digital scan data signal processor of the bar code scanner for conversion into digital words representative of the length of the binary signal levels in the digital scan data signal. This can result in significant errors during bar code symbol decoding operations, causing objects to be incorrectly identified and/or erroneous data to be entered into a host system.
Another drawback to retail laser scanning systems is that the bar code label may be damaged or printed improperly. As is often the case, there being no redundancy in the scanning system, the bar code reader fails to decode and the cashier must input the bar code numbers manually, wasting valuable time at the checkout counter and frustrating customers.
Yet another drawback to retail laser scanning systems is the opportunity for theft by way of scanning the bar code of a significantly less expensive item instead of the item actually passing through the check-out line. Some retailers print their own bar codes to discount certain items. The in-house bar codes are typically printed on stickers and set aside in a bin near the register. Cashiers or customers may peel off these stickers and place them over an existing bar code for an expensive item. As the expensive item is passed over the scan zone, the laser scanner will recognize and decode the less expensive bar code as a valid item, and will complete the transaction at a loss for the retailer. In other fraudulent schemes, cashiers may place the in-house bar code sticker on their hand, and quickly scan their hand instead of the expensive item. Policing such fraudulent actions can be time-consuming and expensive. One method of policing that is presently practiced is to manually review the security camera video at the cashier counter and cross-reference it with sales receipts to assure expensive items (as seen in the video) have been properly transacted. One drawback to this approach is that the theft is identified long after the sale is completed and the customer has left the store.